Network Working Group                                        A. Melnikov
Internet-Draft                                                 Isode Ltd
Updates: 3501 (if approved)                            December 11, 2015
Intended status: Standards Track
Expires: June 13, 2016


           Clarification on IMAP CAPABILITY command behaviour
                  draft-melnikov-imap-capabilities-00

Abstract

   This document clarifies how IMAP (RFC 3501) server implementations
   should handle CAPABILITY command.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on June 13, 2016.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   2
   3.  Clarification on CAPABILITY response/response code  . . . . .   2
   4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   3
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   4

1.  Introduction

   This document clarifies how IMAP [RFC3501] server implementations
   should respond to CAPABILITY command or what they should return in
   CAPABILITY response code at different points in IMAP connection.
   This document updates RFC 3501.

   A CAPABILITY response or CAPABILITY response codes return a listing
   of capabilities that the server supports.  RFC 3501 didn't specify
   whether advertised capabilities can change over time and, if they
   can, at which points in IMAP connection.  This document clarifies
   that.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   The term "IMAP connection" or just "connection" is as specified in
   [RFC3501].

3.  Clarification on CAPABILITY response/response code

   Two successive CAPABILITY commands (with no commands in between them)
   MUST return the same list of capabilities.

   The list of capabilities is generally static, but it can change at 2
   points in IMAP connection ("security state change points"): after a
   successful STARTTLS command and after a successful LOGIN or
   AUTHENTICATE command. () The list of capabilities MUST NOT change at
   any other points.

   With a small number of exceptions, capabilities can't be removed,
   they can only be added or their parameters might change.  Once a
   capability is announced, it can't be taken away in a subsequent



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   CAPABILITY response, except for a few very limited cases.  For
   example, after STARTTLS command is successful, the STARTTLS
   capability doesn't need to be advertised (but it is not an error if
   it is).

   Capabilities that include parameter(s) can change their parameters at
   security state change points.  The later parameter(s) replace any
   previously announced parameters.

   A CAPABILITY response code can contain the same information as a
   CAPABILITY response.  Some implementations only return capabilities
   that apply in non-authenticated state before authentication and only
   capabilities that apply in authenticated/selected state after
   authentication.

4.  Examples

   TBD.  One example: after STARTTLS, AUTH=PLAIN and/or AUTH=EXTERNAL
   can be advertised.

   Second example: Show changing APPENDLIMIT parameter after
   authentication.

5.  IANA Considerations

   This document doesn't require any action from IANA.

6.  Security Considerations

   TBD

7.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
              <http://www.rfc-editor.org/info/rfc3501>.










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Appendix A.  Acknowledgements

   TBD.

Author's Address

   Alexey Melnikov
   Isode Ltd
   14 Castle Mews
   Hampton, Middlesex  TW12 2NP
   UK

   EMail: Alexey.Melnikov@isode.com






































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